This study was designed to test the hypothesis that the frequency response of the systems controlling the motor activity of breathing and walking in quadrupeds is compatible with the idea that supra-spinal locomotor centres could proportionally drive locomotion and ventilation. The locomotor and the breath-by-breath ventilatory and gas exchange (CO2 output (̇ VCO2) and O2 uptake (̇ VO2)) responses were studied in five sheep spontaneously walking on a treadmill. The speed of the treadmill was changed in a sinusoidal pattern of various periods (from 10 to 1 minute) and in a step-like manner. The frequency and amplitude of the limb movements, oscillating at the same period as the treadmill speed changes, had a constant gain with no phase lag (determined by Fourier analysis) regardless the periods of oscillations. In marked contrast, when the periods of speed oscillations decreased, the amplitude (peak-to-mean) of minute ventilation (̇ VE) oscillations decreased sharply and significantly (from 6.1 ± 0.41 min-1 to 1.9 ± 0.21 min-1) and the phase lag between ventilation and treadmill speed oscillations increased (to 105 ± 25° during the 1 min oscillation periods). ̇ VE response followed ̇ VCO2 very closely. The drop in ̇VE amplitude ratio was proportional to that in ̇ VCO2 (from 149 ± 48 ml min-1 to 38 ± 5 ml min-1) with a slightly longer phase lag for ventilation than for ̇ VCO2. These results show that beyond the onset period of a locomotor activity, the amplitude and phase lag of the ̇ VE response depends on the period of the walking speed oscillations, tracking the gas exchange rate, regardless of the amplitude of the motor act of walking. Locomotion thus appears unlikely to cause a simple parallel and proportional increase in ventilation in walking sheep.
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